Generally, the widely-used peripheral input device of a computer system includes for example a mouse device, a keyboard device, a trackball device, or the like. Via the keyboard device, characters or symbols can be inputted into the computer system directly. As a consequence, most users pay much attention to the keyboard devices.
FIG. 1 is a schematic side view illustrating a portion of a conventional keyboard device and taken along a viewpoint. The conventional keyboard device 1 comprises a base plate 13, a membrane circuit board 14 and plural key structures 12. Each of the plural key structures 12 comprises a keycap 121, a scissors-type connecting element 122 and an elastic element 123. The scissors-type connecting element 122 is connected between the keycap 121 and the base plate 13. The scissors-type connecting element 122 comprises a first frame 1221 and a second frame 1222. The second frame 1222 is pivotally coupled to the first frame 1221. Consequently, the first frame 1221 and the second frame 1222 can be swung relative to each other. The elastic element 123 is arranged between the keycap 121 and the membrane circuit board 14. Moreover, the elastic element 123 comprises a contacting part 1231.
The membrane circuit board 14 comprises an upper film layer 142, a lower film layer 141 and an intermediate film layer 143. The intermediate film layer 143 is arranged between the upper film layer 142 and the lower film layer 141. A first circuit pattern 1421 is formed on a bottom surface of the upper film layer 142. The first circuit pattern 1421 comprises plural upper silver paste conductor lines 14212 and plural upper contacts 14211. The upper contacts 14211 are aligned with the plural key structures 12, respectively. A second circuit pattern 1411 is formed on a top surface of the lower film layer 141. The second circuit pattern 1411 comprises plural lower silver paste conductor lines 14112 and plural lower contacts 14111. The plural lower contacts 14111 are aligned with the plural upper contacts 14211, respectively. In addition, the intermediate film layer 143 comprises plural perforations 1431 corresponding to the plural upper contacts 14211 and the plural lower contacts 14111. Each of the upper contacts 14211 and the corresponding lower contact 14111 are collectively defined as a membrane switch 144.
While the keycap 121 of any key structure 12 is depressed and moved downwardly relative to the base plate 13, the first frame 1221 and the second frame 1222 of the scissors-type connecting element 122 are switched from an open-scissors state to a stacked state. Moreover, as the keycap 121 is moved downwardly to compress the elastic element 123, the corresponding upper contact 14211 is contacted with and triggered by the contacting part 1231 of the elastic element 123. Consequently, the corresponding upper contact 14211 is penetrated through the corresponding perforation 1431 and contacted with the corresponding lower contact 14111. Under this circumstance, the corresponding membrane switch 144 is electrically conducted. When the keycap 121 of the key structure 12 is no longer depressed, the keycap 121 is moved upwardly relative to the base plate 13 in response to an elastic force of the elastic element 123. Meanwhile, the first frame 1221 and the second frame 1222 are switched from the stacked state to the open-scissors state again, and the keycap 121 is returned to its original position.
FIG. 2 is a schematic side view illustrating a portion of the membrane circuit board of the conventional keyboard device as shown in FIG. 1 and taken along another viewpoint. FIG. 3 is a schematic top view illustrating a portion of the membrane circuit board as shown in FIG. 2. The lower film layer 141 of the membrane circuit board 14 further comprises an extension part 1412. Moreover, at least some of the lower silver paste conductor lines 14112 of the second circuit pattern 1411 are formed on the extension part 1412 as plural pins. Moreover, plural carbon ink protective layers 14113 are formed over respective pins to protect the lower silver paste conductor lines. When these pins are inserted into a connector (not shown), the connector may pierce the carbon ink protective layers 14113 to contact the lower silver paste conductor lines 14112. Consequently, the electric connection is established.
However, the membrane circuit board 14 and the keyboard device 1 with the membrane circuit board 14 still have some drawbacks. For example, in case that the sulfur-containing air enters the membrane circuit board 14 and contacts with the upper silver paste conductor lines 14212 of the first circuit pattern 1421 and the lower silver paste conductor lines 14112 of the second circuit pattern 1411, the upper silver paste conductor lines 14212 and the lower silver paste conductor lines 14112 are gradually turned into silver sulfide. Consequently, the conductive impedance is gradually increased, and the upper silver paste conductor lines 14212 and the lower silver paste conductor lines 14112 are unable to transfer electronic signals.
Since the lower silver paste conductor lines 14112 and the carbon ink protective layers 14113 are formed through different fabricating processes, it is difficult to align the lower silver paste conductor lines 14112 with the carbon ink protective layers 14113. If any pin is deviated from the corresponding carbon ink protective layer 14113, the electric connection between the membrane circuit board 14 and the connector is adversely affected.
Consequently, the conventional membrane circuit board and the conventional keyboard device with the membrane circuit board need to be further improved.